Street Rod Engine Blowers – Blower Basics

Building A Supercharged Engine For The Street

To paraphrase an oft-repeated line in one of our favorite Westerns, there are two kinds of street rod engines in the world, my friend-those with blowers, and who cares what the others are. The point is, you can pretty much forget about every other feature on a car when it’s got a huffer hung on the engine because the obvious statement being made is that nothing else matters much anyhow. Compared to an over-teched engine covered with something that resembles an upside-down wheelbarrow, a blower adds a visual testosterone factor that is unmatched. But the real beauty of a blower is the performance potential it provides.

Internal combustion engines are basically an air pump, and in normal circumstances, the only thing that makes the cylinders fill with fuel and air on the intake stroke is atmospheric pressure. How well that is done is described as volumetric efficiency (VE), which is the difference between the theoretical maximum amount of air (or air and fuel) each cylinder can take in during the intake cycle compared to the actual amount taken in. For contemporary, naturally aspirated (unblown) engines, a VE over 95 percent is excellent, 100 percent is possible, and some competition engines can exceed 110 percent. Now, let’s try an admittedly crude analogy on what this means (engineers need not send any hate mail). Think of a 350ci engine running at 90-percent efficiency; the result is a powerplant that performs like it has 10-percent less displacement, or 315 ci. Now take that same engine and add a blower. As mixture is forced into the cylinders, it’s possible to get a denser air/fuel mixture into the cylinders and exceed 100-percent volumetric efficiency. In fact, just 6 or 7 lbs of boost can result in a 40- to 50-percent increase in VE; at 140 percent, that same 350 is now being stuffed with the equivalent of 490 ci of fuel and air. (Does that make it a little easier to understand while all those turbo-charged, small-displacement imports haul buns?).

While there are a variety of supercharger types-exhaust-driven turbos, engine-powered centrifugals, and so on-the GMC Roots type has historically been the most popular with hot rodders. Originally used on two-stroke Diesels to blow fresh air in and exhaust out of the cylinders, they are identified by a numbering system that indicates their origin. As examples, a 4-71 was fitted to an inline-four-cylinder Diesel with 71 ci per cylinder, and a 6-71 equates to an inline-six-cylinder engine with 71 ci per cylinder. A variation on the theme was the GMC blower used on V-type engines; a 6V-92 means the engine is V-6 with 92 ci per cylinder, and the 8V-71 designation refers to a V-8 with 71 ci per cylinder. Another method of identifying blowers is by displacement, or how much air will be moved by one revolution of the rotors. A 144 blower would move that many cubic inches of mixture with one revolution of the rotors.

Fact Versus FictionBecause blowers are most often identified with Top Fuelers and other seriously fast race cars, many rodders assume that blown engines are difficult to drive on the street, unreliable, and are gas guzzlers. The truth is, a properly prepared blown engine is a treat to live with. First off, they start immediately because the blower fills the cylinders as soon as the engine begins to spin, and once running, throttle response is amazing while the power is delivered in a very linear and controllable manner. From a maintenance standpoint, a Roots-type blower will probably outlast the engine it’s mounted on. And while no one builds a blown engine with economy as the primary goal, roughly a 3-percent drop in mileage is the norm, unless you’ve got the throttle pedal mashed to the floor constantly, in which case all bets are off.

Another misconception about Roots blowers concerns their basic operation. Air does not travel down between the rotors; rather it is pushed around the outside of the case and into the manifold. And it is there in the manifold, not in the blower case, that boost is created.

Speaking of boost, a Roots blower will only provide boost when the air is available; that means boost is a function of throttle position. At small throttle openings, the volume of air is reduced and as a result, there is little or no boost. As the throttles open, more air is available and the boost increases. Of course maximum boost is dependent upon the size of the blower, the drive ratio, and a variety of other factors.

Blower SelectionWhile there are a host of Roots-style blowers on the market, in our particular case, the engine involved-a 392 Chrysler-had a great deal to do with our choice. From a purely aesthetic standpoint, the blower had to be big enough physically to look at home on a Hemi. From a practical standpoint, it had to be reliable as an anvil. For those reasons we elected to use a 6-71 blower and kit from Weiand. It came complete with the blower, manifold, drive pulleys, idler and bracket, and linkage. We also opted for a pair of Holley carburetors and their trick air scoop and filters (more on them later).

For Chrysler Hemis, Weiand offers two blowers-the 6-71 and the 8-71. And while the obvious difference is size, they also differ in design internally. The original GMC blowers, and many aftermarket designs including the Weiand 8-71, use a pair of three-lobe rotors that have some helix, or twist, built into them. A variation on the Roots theme are those blowers, Weiand’s 6-71 and many others that use a pair of straight, two-lobe rotors. As you might guess, there are arguments put forth for both designs. Two-lobe rotors are said to move more air per revolution, and the straight rotors don’t push the air to one end of the case like the three-lobe, helix design rotors do. On the other side of the debate are those who say that two-lobe rotors create pulses in the intake manifold and they allow internal leakage that reduces boost. The fact is all these arguments have merit.

When choosing a blower, there is a simple guideline to follow: A smaller engine with a lower rpm boost required warrants a smaller blower, while a bigger engine with a higher rpm boost required calls for a bigger blower. In our case, we had a moderately sized engine (392 ci) that wouldn’t see lots of rpm (we have a self-imposed 6,000-rpm redline, and probably won’t spin it that tight) and plan on boost in the 5- to 7lb range. We decided that for our application the two-rotor 6-71 would work just fine; had we wanted a higher level of performance and more boost, we would have selected the three-lobe 8-71.

Boost LevelsThe biggest factor determining how much boost can be run is the engine’s compression ratio. Generally speaking, the static compression ratio of the engine being supercharged should be 9.0:1, or preferably less. Higher compression than that will severely limit the amount of boost the engine will tolerate. On the other hand, much less than 7.0:1 will result in an engine that is “lazy” when no boost is present; we split the difference with our engine and have a static compression ratio of 8.0:1.

Check out the chart below; static compression is down the left side with boost across the top. Our plan is to run 4 to 6 lbs of boost, which will result in a maximum effective compression ratio of 10.2:1 to 12.4:1 (we’ll experiment to see what the engine will tolerate). While a Hemi is somewhat forgiving in this regard, effective compression ratios of more than 12:1 are an invitation for disaster in the form of detonation.

Engine ModificationsAs we’ve said, we’re working with a Chrysler Hemi, but the modifications necessary to supercharge any engine are about the same, and again we’ll put things in simple terms: The more boost and the more demands placed on the engine, the more modifications are necessary. For low-boost applications, 3 to 4 lbs, a sound, stock engine is about all you need; cast pistons will even live. Upping the boost to 5 to 9 lbs will call for the next level of modifications and 10 or more will call for some serious preparation. But let’s look at it from a component standpoint.

BlockTry to keep any overbore to a minimum to preserve cylinder-wall integrity, and investing in sonic checking isn’t a bad idea if you have doubts about the thickness of the cylinder walls. Two-bolt mains are adequate on the street, although main studs are a wise addition. If four-bolt main blocks are available for your application, they’re advisable, and for more than 10 lbs of boost, they should be added to any block if it’s feasible.

CrankshaftsCast crankshafts will suffice in low-boost, low-rpm situations, but forged cranks are generally preferred when boost rises much past 8 lbs or the tachometer goes beyond six grand.

RodsMost factory rods are good to 8 to 10 lbs of boost, but as a precaution, they should be fitted with aftermarket bolts. However, keep in mind many aftermarket offerings are now available for not much more that the cost of rebuilding a set of stockers. Full-floating piston pins are recommended.

PistonsWhile factory cast pistons will work at low-boost levels, forged pistons are a better choice due to their strength and the ability to handle elevated combustion temperatures (which happens with more mixture in the cylinders). An alternative with moderately boosted engines are hypereutectic pistons; they split the difference between cast and forged pistons in both price and strength categories.

HeadsBecause the blower is doing the work in terms of filling the cylinders, the heads don’t normally require significant modifications. The real advantage to a blower on a street engine is the increased bottom and midrange power, so the heads don’t have to flow a tremendous amount of air that high-rpm operation requires. The trick here is to build the engine so it has a broad power and gear the car to take advantage of it. Again, if all-out performance is the goal, bigger ports and valves will likely provide power gains, particularly in high-rpm/boost applications.

CamsThis is a subject unto itself and one where there is much debate. The first decision is solid versus hydraulic, and for all practical purposes, the same arguments apply that are used for these cams in normally aspirated engines. Hydraulics require less maintenance than solids in terms of periodic adjustments; however, solids will raise the engine’s redline. The same goes for flat tappets versus rollers, and all the prevailing arguments apply to which is better.

The real difference in cams for blower motors has to do with lobe centers, or the separation between the intake and exhaust lobes. Wide-lobe centers, 112 to 114 degrees, spread the valve events apart, which means less overlap (during the period when the exhaust valve is not quite closed and the intake is just beginning to open, so both valves are in fact open) and results in more cylinder pressure. Of course, that’s just another way of saying there’s more stuff in the cylinder to burn, and that makes more heat. So here’s the deal: Wider lobe centers equal more power and more heat; closer lobe centers equal less power, but a cooler-running engine with less chance of detonation.

Another area of cam design for blown engines that is often discussed is duration. Many engine builders favor a split-pattern cam, which is another way of saying a cam with more duration on one valve event than the other. For blown engines, there is often more duration on the exhaust because the blower is doing the work filling the cylinders on the intake stroke more effectively, so more duration on the exhaust just gives the piston more time to clear the spent gasses from the cylinder.

Finally, a blower has a tendency to calm a radical cam, so you can get away with more duration on the street than you could with a naturally aspirated engine. The bottom line is, a blower makes up for a lot of an engine’s shortcomings and even a relatively stock cam will work; more lift and longer duration just give the blower more opportunity to fill the cylinders.

Vibration Dampers Versus HubsA vibration damper does just that-it cancels out the harmonics that occur in the crankshaft as the power produced in the cylinders is applied to it. In most cases, the vibration damper also mounts a pulley that drives the engine’s accessories, such as the water pump, alternator, etc. The problem is that most stock vibration dampers aren’t strong enough to withstand the stress of driving a blower as well.

One way to deal with that issue is to replace the vibration damper with a solid steel hub. Oddly enough, the blower and belt tend to do the same thing as a vibration damper, just not as well. We prefer to use a vibration damper that is designed for the task of mounting pulleys and driving a blower.

InductionNo matter what kind of system is in use-carburetors or fuel injection-a blown engine has the capacity to use lots of fuel and the last thing you want to do is run it too lean. And while there are lots of variables due to engine displacement, type of induction, and so on, a high-capacity fuel pump (or in some cases, pumps) and large lines should always be part of the package.

While there are a host of carburetor modifications that are often made, one of the most common on Holley carburetors is what’s called a boost-referenced power valve. In part-throttle conditions where the carburetor isn’t exposed to enough vacuum to open the power valves but the blower is providing boost, the engine will often exhibit a lean surge. By using boost pressure to control the power valve(s) rather than vacuum, that tendency is eliminated.

IgnitionAll performance engines need a strong spark; those with blowers are no different. However, a blown engine does have unique needs in terms of the advance curve. Blown engines like lots of initial advance and often a little less total advance than a normally aspirated engine needs.

ExhaustIt stands to reason if a blower puts more air into the engine, there’s more that has to get out. That means, in most cases, 2 1/2 inches is the minimum exhaust system size, and 3 inches is probably better when the boost gets into double digits.

Cooling SystemIt’s this simple-more power equals more heat. Cram in as much radiator as possible, run the biggest fan that will fit, use a shroud and a recovery tank. Oh, and you can probably forget the smooth look with no louvers in the hood. Airflow is often the key to keeping an engine cool.

Putting It All TogetherWhen it came time to build our blown Hemi, we relied on two of our go-to guys: John Beck of Pro Machine, for the prep work and assembly, and Bob Walker of Hot Heads, for the right parts and a wealth of Hemi information.

Our intent here is to show what it takes to build a blown engine, and since most or our readers have seen photos of machine work being done, we’ll skip all that. Suffice it to say, Beck made sure the block was square with the world in every conceivable direction; the crank was turned, indexed, prepped, polished, and a few other things we probably forgot. Everything that moved was balanced to a gnat’s eyelash and the heads had the ports mildly massaged; the valves and seats were ground and new springs were installed. The really trick parts we used, namely the rods and oil pan, along with most of the internal components, came from Hot Heads, and all the blower-related components can be found in Weiand’s catalog. In short, there’s nothing here that can’t be easily duplicated.

Of course the burning question at this point is, how much horsepower and torque did the engine make? To find out, you’ll have to wait until next month; we’ll show you all the numbers and explain how to tune a blown engine (specifically, much more on ignition and induction). We promise it will be worth the wait.

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